1. Field of the Invention
This invention relates generally to an apparatus for molding or retreading the tread stock of a tire, and more specifically, to a system for readily replacing sipe blades in such an apparatus with those having a different configuration, for replacing worn or broken sipe blades, or for eliminating a sipe blade altogether.
2. Description of the Related Art
Molds and retread presses are types of equipment that are used to form features such as grooves, recesses, tread blocks, and sipes or lamellas on the tread stock of a tire. Sipes or lamellas are thin slits in the tread stock of a tire that enhance certain characteristics of the tire such as handling. Sipes are formed by thin projections or blades that extend from a curing surface of a mold or of a plate found in a retread press. In a molding situation, a new tire is placed in the mold and then the mold sectors which form the tread stock of the tire move in until projections such as sipe blades form the desired geometry on the tread stock. When forming a sipe, the blade penetrates the tread stock as the mold sectors move toward the tire. Once the mold sectors have moved completely into a closed position, the curing surfaces of the mold sectors are in contact with the tread stock of the tire and the top portion of the mold is closed such that the complete tire is encapsulated within the mold. The mold then supplies heat to the tread stock partially by conduction through the curing surfaces of the mold sectors to the exterior of the tire while heat is conducted to the interior of the tire via the membrane. This heats the rubber of the tire until it vulcanizes, leaving the geometry permanently embossed on the tire.
On the other hand, the retreading process is used to replace the tread on a used tire. First, the worn tire tread stock is removed from the tire. Second, new tire tread stock with the proper geometry is formed by placing a flat piece of tread stock in a retread press that has sipe blades and other projections found on a curing plate that has been installed into the press. The press is then closed until the sipe blades and other projections engage the tread stock and the curing plate presses up against the tread stock. Sometimes sipe blades are located on the top portion of the retread press when the sipes are intended to open up to the interior of the tire. Other times the blades are found on the bottom portion of the press when the sipes are intended to face toward the exterior of the tire. Heat is then conducted to the tread stock from both the bottom and top plates of the press until the rubber vulcanizes, leaving the geometry permanently embossed on the tread stock. Finally, the new tread stock is attached to the circumference of the tire.
When forming sipes on tread stock, regardless of whether it is by the molding or retreading processes, the sipe blades that form the sipes are thin and subject to repeated stress. Accordingly, these blades can become worn or broken. Therefore, there is a need to replace worn or broken blades with new blades. Also, different types of tires have different geometry on their tread stock with different features necessitating that sipe blades with different configurations be used. Also, the pattern in which sipe blades or other projections are arranged needs to be changed to produce different types of tires. As a result, there has also been a need to mold and retread tread stock with different features. One way to accomplish this is to have dedicated molds and curing plates with sipe blades and other projections permanently attached to them so that different types of tires can be manufactured. However, it is often cost prohibitive to make a dedicated mold sector or curing plate for every type of tire, especially in situations where a certain type of tire is produced in limited volumes. In such situations, it is preferable to have a mold sector or curing plate that can be changed over from one configuration to another, so that different features and/or sipe blade patterns and configurations can be embossed onto a tread stock using essentially the same apparatus. Therefore, it is desirable to have a system for molding or retreading tires that allows such a changeover. Finally, it would be desirous to provide a system that creates this changeover in a foolproof manner, prohibiting the assembler from creating incorrect geometry for producing a particular tire that would result in scrap and lost profits. Fool proofing can also prevent mold and press crashing caused by components of one side of an apparatus hitting the other side of the apparatus because components are improperly oriented or located, which is an undesirable expense.
Sipe blades come in two different basic configurations. The first type is called a two dimensional sipe blade, so called because its geometry varies in a plane that is parallel to the curing surface of a curing plate in the retreading application, or a plane that is perpendicular to the radius of the tire in a molding application. The geometry of a two dimensional sipe blade does not vary or is straight in the direction of draw for the sipe blade. The direction of draw is the direction a sipe blade moves to withdraw from the tread stock after the sipe has been formed. In the molding process, the draw direction is in a generally outward radial direction of the tire. In the retreading process, the draw direction is perpendicular to the curing surface and away from the tread stock. The second type of sipe blade is a three dimensional sipe blade and has geometry that varies both in a plane that is parallel to the draw direction and a plane that is perpendicular to the draw direction. An undercut is formed by a three dimensional sipe blade because of its geometrical variation in the direction of draw which can result in a larger force being necessary to withdraw the three dimensional sipe blade from the tread stock.
For reference, the spatial relationship between different features in this specification and the claims will be measured in the anti-draw direction which is parallel and opposite to the draw direction (see FIG. 2 which shows the anti-draw direction as Arrow A). Accordingly, features that are located further in the anti-draw direction than others will be referred to as being “above” them. Likewise, features that are located further in the draw direction than others will be referred to as being “below” them. Similarly, the surface of a feature that is located furthest in the anti-draw direction will be referred to as being the “top” surface. On the other hand, the surface of a feature that is located furthest in the draw direction will be referred to as being the “bottom” surface.
FIGS. 1 and 2 disclose an apparatus that attempts to satisfy some of the aforementioned needs. Although it involves the use of a flat retreading press, it is to be understood that this apparatus could be easily modified to be used with a round mold for making a new tire or retreading an existing tire as well. This apparatus comprises a curing plate 50 that has a slit 52 formed in it using a wire EDM process. The slit 52 is configured to be complimentary to the shape of a two dimensional sipe blade 54 that is to be inserted through the bottom surface 56 of the curing plate 50 until the molding portion 58 of the sipe blade 54 has extended through the slit 52 and rises above the curing surface 60. The gap between the blade 54 and the slit 52 is about six and a half hundredths of a millimeter on a side of the blade 54 on average in order to prevent the rubber from flashing into the recess during curing. As can be seen, the blade 54 has two heels 62 that extend from its retention portion 64, which contact the bottom 56 of the curing plate 50, preventing the sipe blade 54 from passing through the curing plate 50. A piece of the retention portion 64 of the sipe blade 54 is cut out, separating the two heels 62 and forming a clamp surface 66 that is found above the bottom surface 68 of the heels 62 and which is coplanar with top surface 70 of the heels 62. Hence, the clamp surface 66 of the sipe blade 54 is flush with the bottom surface 56 of the curing plate 50 when the top surfaces 70 of the heels 62 contact the bottom surface 56 of the curing plate 50, leaving only the heels 62 extending below the curing plate 50. A retainer plate 72 with apertures 74 configured to clear the heels 62 is mounted to the curing plate 50, pressing onto the clamp surface 66 of the sipe blade 54 and capturing it between both plates.
This design allows two dimensional sipe blades 54 to be removed when worn or broken by simply disconnecting the retainer plate 72 from the curing plate 50 and pulling the sipe blade 54 back out of the slit 52. However, this design has several drawbacks. First, the slit 52 is wired directly into the curing plate 50 which means only another sipe blade 54 that has the same two dimensional variation in its geometry can be used in that slit 52. So this design does not allow a changeover to another sipe blade 54 having another configuration. Second, this design does not easily allow for a sipe blade 54 to be eliminated as the slit 52 will allow rubber to seep into it when no sipe blade 54 is present. Third, this design does not work for three dimensional sipe blades since the molding portions of these blades are larger than those of two dimensional sipe blades 54 and they cannot fit through the small slit 52 that accommodates two dimensional sipe blades 54.
Accordingly, there still exists a need for a system that allows all sipe blade configurations, including two and three dimensional, to be changed out for sipe blades having other configurations in molding or retreading processes and to selectively eliminate a sipe blade if so desired.